config: add replaceStdenv, cudaCapabilities, cudaForwardCompat options (#479234)

# Select the appropriate stages for the platform `system'.

if crossSystem != localSystem || crossOverlays != [ ] then

  stagesCross

else if config ? replaceStdenv then

# The `or null` fallback is needed for contexts that don't use the module system (e.g. tarball builds).

else if (config.replaceStdenv or null) != null then

  stagesCustom

else if localSystem.isLinux then

  stagesLinux

      feature = "build packages with CUDA support by default";

    };

    cudaCapabilities = mkOption {

      type = types.listOf types.str;

      default = [ ];

      description = ''

        A list of CUDA capabilities to build for.

        Packages may use this option to control device code generation to

        take advantage of architecture-specific functionality, speed up

        compile times by producing less device code, or slim package closures.

        For example, you can build for Ada Lovelace GPUs with

        `cudaCapabilities = [ "8.9" ];`.

        If not provided, the default value is calculated per-package set,

        derived from a list of GPUs supported by that CUDA version.

        See the [CUDA section](https://nixos.org/manual/nixpkgs/stable/#cuda) in

        the Nixpkgs manual for more information.

      '';

    };

    cudaForwardCompat = mkOption {

      type = types.bool;

      default = true;

      description = ''

        Whether to enable PTX support for future hardware.

        When enabled, packages will include PTX code that can be JIT-compiled

        for GPUs newer than those explicitly targeted by `cudaCapabilities`.

      '';

    };

    replaceBootstrapFiles = mkMassRebuild {

      type = types.functionTo (types.attrsOf types.package);

      default = lib.id;

      '';

    };

    replaceStdenv = mkMassRebuild {

      type = types.nullOr (types.functionTo types.package);

      default = null;

      defaultText = literalExpression "null";

      description = ''

        A function to replace the standard environment (stdenv).

        The function receives an attribute set with `pkgs` and should return

        a stdenv derivation.

        This can be used to globally replace the stdenv with a custom one,

        for example to use ccache or distcc.

      '';

      example = literalExpression "{ pkgs }: pkgs.ccacheStdenv";

    };

    rocmSupport = mkMassRebuild {

      feature = "build packages with ROCm support by default";

    };